Many electrophotographic machines, such as photocopiers or laser printers, use a detachable developer cylinder that contains a photosensitive member. The cylinder is also known as a cartridge or developer cartridge, and it is detachably mounted to the complete copier or printer. This construction enables users to maintain the printing capability of the device without the aid of a serviceman.
The developer cylinder must be coupled to a drive mechanism that rotates the cartridge. Typically this is accomplished by a driving member with a hole or recess that couples with a projecting end in the driven member, which includes the cartridge. U.S. Pat. No. 5,987,287 to Huang, which is incorporated by reference into this disclosure, is a typical attempt to address the problems of connecting the developer cylinder. FIG. 1 of Huang depicts a typical prior art coupling and is the reference on which the following description is based. The drive coupling includes a drive shaft B that is centered in drive gear A that, in turn, drives developer cylinder E. The end of developer cylinder E has a fixedly mounted a flange that includes a transmission gear F and triangular coupling block D that fits in coupling recess C, which is disposed at the end of drive shaft B. Recess C is a twisted triangular hole, while coupling block D is a twisted triangle that is engageable with recess C. The driven end of the cylinder is typically made of plastic. Although not shown in Huang, many prior art couplings, including those in commercial use, include some form of pin and hole that assist the coaxial alignment of the driving and driven portions of the coupling.
Because the torque on twisted coupling block D of Huang is concentric to its three pointed vertices, the driven cylinder flange with its projection and the driving mechanism both encounter high stresses. An example of this problematic arrangement can be seen in FIG. 18 of U.S. Pat. No. 5,903,803 to Kawai et al. In FIG. 18a a triangular projection fits in a larger triangular recess without the two pieces engaging. FIG. 18b depicts the relative rotation of the two pieces such that they engage where the tips of the smaller triangle meet the flat sides of the larger recess. The relative sizes of the projection and recess, and the lack of precise engagement, occur because replacing the developer cartridge requires some dimensional tolerance. Many existing machines use a triangular or twisted triangular recess as the driving mechanism and some form of triangular driven mechanism.
In an effort to reduce the stress where the driving and driven portions of the coupling engage, some have tried beveling the triangular tips in an attempt to achieve a greater contact area between the triangular tips and the sides of the recess into which it fits. See, e.g., Kawai FIGS. 19a and 19b. Other examples of attempted solutions to the coupling problem can be seen in U.S. Pat. No. 7,433,631 to Karz et al; U.S. Pat. No. 6,768,890 to Cho et al; U.S. Pat. No. 6,173,146 to Wang et al; and, U.S. Patent Appl. No. 2010/0196047 to Jin. Karz uses a triangular recess that engages with three skewed blades mounted on a rod. Cho uses a tapered recess with spiral protrusions that engages with a tapered shaft that has spiral sloping ribs. Wang et al describes a triangular recess coupling with a shaft that has a plurality of teeth spaced spirally around the shaft. Wang has circular spiral teeth, not the more adaptable and effective helical involute teeth of the present invention. Jin emphasizes the tradeoffs between accuracy and wear, highlighting that his invention involves point rather than surface contact. According to Jin, that arrangement—point contact—makes torque transmission more mechanically reliable, which in turn makes the printing more accurate. The point contact, however, hastens the wear of the driving and driven mechanisms because of the stress concentration at the point. Jin is also more likely to damage the recess of the driving mechanism, which affects the coaxial alignment of the driving and driven mechanisms and thus causes the print quality to deteriorate as the coupling wears. If the wear to the recess of the driving mechanism is too great, the driving mechanism could require replacement. This part of the device is much more difficult to remove and repair than simply replacing a plastic cartridge that contains a developer cylinder with ink or toner.
During the life of a device such as a printer or facsimile, the developer cartridge may be replaced many times, depending on the design of the device and the user's printing needs. A successful mechanical coupling must be designed in a way to 1) maximize the torque that can be transferred from the driving mechanism to the driven mechanism and the developer cylinder; 2) maintain the coaxial alignment of the driving and driven mechanism, which also maintains the accuracy of the printing process; 3) minimize the wear on the driving and driven mechanisms, which in turn maintains the accuracy and reduces the maintenance cost of the machine; 4) maintain the coupling's dimensional tolerances to facilitate easy replacement of the cartridge with the developer cylinder; and, 5) render a low manufacture cost. Despite the numerous designs of mechanical couplings, both for devices like photocopiers and facsimiles as well as for other types of mechanical couplings, there remains a need for an improved rotational coupling that will overcome all of these problems.